1. Field of the Invention
The present invention relates to a high frequency detection device that detects a high frequency signal of high frequency power transmitted through a coaxial line. The present invention particularly relates to a coaxial cable including such a high frequency detection device.
2. Description of Related Art
A plasma processing system has heretofore been developed in which high frequency power is supplied to a plasma processing apparatus so as to process a workpiece such as a semiconductor wafer or a liquid crystal substrate.
FIG. 12 is a block diagram showing the configuration of a generally used plasma processing system. The impedance of a plasma processing apparatus 300 varies during plasma processing. Accordingly, the reflected wave power reflected at an input terminal of the plasma processing apparatus 300 may cause damage to a high frequency power source apparatus 100. To address this, an impedance matching apparatus 200 is provided between the high frequency power source apparatus 100 and the plasma processing apparatus 300 so that the impedance matching apparatus 200 can perform a matching operation according to the impedance variation of the plasma processing apparatus 300.
The system described above needs to monitor the impedance of the plasma processing apparatus 300 during plasma processing, as well as a high frequency voltage and a high frequency current at the input terminal of the plasma processing apparatus 300. Monitoring of the plasma processing apparatus 300 is performed using various types of high frequency parameters measured by a high frequency measurement apparatus 500 disposed on a transmission line 400 connecting the impedance matching apparatus 200 and the plasma processing apparatus 300 at a position close to the plasma processing apparatus 300. A conventional current/voltage detector is disclosed in JP 2009-36553A or JP 2009-58449A.
The high frequency measurement apparatus 500 detects a high frequency voltage signal and a high frequency current signal, and determines a phase difference θ between high frequency voltage and high frequency current from the detected signals. The high frequency measurement apparatus 500 also calculates high frequency parameters including a voltage effective value V, a current effective value I, an impedance Z=R+jX (corresponding to the impedance of the plasma processing apparatus 300 because the measurement point is near the input terminal of the plasma processing apparatus 300), a reflection coefficient Γ, a traveling wave power Pf input into the plasma processing apparatus 300, a reflected wave power Pr reflected at the input terminal of the plasma processing apparatus 300 due to impedance mismatch, and the like.
The high frequency measurement apparatus 500 includes a high frequency detection device 510 that is disposed on the transmission line 400 and detects a high frequency voltage signal and a high frequency current signal, and a computing unit 520 that calculates various types of high frequency parameters through computation from the high frequency voltage signal and the high frequency current signal that have been detected by the high frequency detection device 510.
FIG. 13 is a diagram illustrating a general internal configuration of the high frequency detection device 510. As shown in FIG. 13, the high frequency detection device 510 includes a power transmission body 511, a current transformer portion 512, a current conversion circuit 513, a capacitor portion 514, and a voltage conversion circuit 515.
The power transmission body 511 is connected to an inner conductor of the transmission line 400 and transmits the high frequency power output by the high frequency power source apparatus 100. The power transmission body 511 can be, for example, a conductor such as a cylindrical rod made of copper, and its outer surface is covered with an insulator. The current transformer portion 512 detects a current according to the high frequency current flowing thorough the power transmission body 511 and outputs the detected current to the current conversion circuit 513. The current conversion circuit 513 converts the input current to a high frequency current signal, which is a predetermined voltage level signal, and outputs the signal to the computing unit 520. The capacitor portion 514 detects a voltage according to the high frequency voltage generated in the power transmission body 511 and outputs the detected voltage to the voltage conversion circuit 515. The voltage conversion circuit 515 converts the input voltage to a high frequency voltage signal, which is a predetermined voltage level signal, and outputs the signal to the computing unit 520. The computing unit 520 receives input of the high frequency current signal and the high frequency voltage signal from the high frequency detection device 510, calculates various types of high frequency parameters through computation, and outputs the parameters.
If there are variations in the shapes of the current transformer for current detection, the capacitor for voltage detection and wires, variations will occur in the detected values output from the high frequency detection device 510. In order to suppress such variations, technology has been developed for forming the current transformer, the capacitor and wires on a printed circuit board in the form of printed wiring.
FIG. 14 is a diagram illustrating a high frequency detection device 510 that uses a printed circuit board in which the current transformer, the capacitor and wires have been formed in the form of printed wiring. As shown in FIG. 14, the high frequency detection device 510 includes the power transmission body 511, a current detection printed circuit board 516, a voltage detection printed circuit board 517, and a housing 518.
The current detection printed circuit board 516 is a printed circuit board in which the current transformer portion 512 and the current conversion circuit 513 included in the internal configuration of the high frequency detection device 510 shown in FIG. 13 and a wire for outputting the current detected by the current transformer portion 512 to the current conversion circuit 513 have been formed. Likewise, the voltage detection printed circuit board 517 is a printed circuit board in which the capacitor portion 514 and the voltage conversion circuit 515 included in the internal configuration of the high frequency detection device 510 and a wire for outputting the voltage detected by the capacitor portion 514 to the voltage conversion circuit 515 have been formed. The housing 518 fixes the current detection printed circuit board 516 and the voltage detection printed circuit board 517, and protects the substrates 516 and 517 from external electromagnetic waves and the like. The housing 518 can be made of, for example, a conductor such as aluminum.
FIGS. 15(a) to 15(c) are diagrams illustrating an example of the voltage detection printed circuit board 517. As shown in FIG. 15(a), in the voltage detection printed circuit board 517, the voltage conversion circuit 515 is mounted on a substrate, and predetermined printed wiring is formed. Also, the substrate has a penetration hole formed therein, and the power transmission body 511 (see FIGS. 13 and 14) is disposed in the penetration hole. A ring-shaped wiring 514a constitutes the capacitor portion 514 (see FIG. 13) between the ring-shaped wiring 514a and the power transmission body 511, and is ring-shaped wiring formed along the periphery of the penetration hole. FIGS. 15(b) and 15(c) are diagrams illustrating the ring-shaped wiring 514a. FIG. 15(b) is an enlarged view of a region surrounded by a broken line and indicated by c in FIG. 15(a), and FIG. 15(c) is a cross-sectional view taken along the line D-D′ in FIG. 15(b). As shown in FIGS. 15(b) and 15(c), the ring-shaped wiring 514a is formed by through holes passing through the substrate and printed wiring connecting the through holes. Generally, a through hole refers to a connecting means in which a conductor layer (for example, copper) is provided inside a penetration hole formed in a substrate. Such a through hole includes a type for insertion of a lead wire of an electronic component and a type intended only for establishing an electrical connection between the surface and the back surface of a substrate. The latter type of through hole is particularly called a “via hole”. The via hole includes a through type (through via) that extends from the surface to the back surface of a substrate and an interstitial via hole that extends only partially through a multi-layer substrate. Furthermore, the interstitial via hole includes a blind via through which the hole can be seen from one side of a substrate and a buried via that has been completely buried in a substrate. In this specification, the term “through hole” is used to mean both “through via” and “interstitial via hole”.
The voltage conversion circuit 515 includes a capacitor C2 connected in series to the capacitor portion 514 (hereinafter also referred to as the “capacitor C1” depending on the case) and a resistor R1 connected to a connection point between the capacitor C1 and the capacitor C2. The voltage detection printed circuit board 517 divides the high frequency voltage generated by the power transmission body 511 disposed so as to pass through the penetration hole between the capacitor C1 and the capacitor C2, adjusts the voltage level of the divided voltage generated at the connection point between the capacitor C1 and the capacitor C2 using the resistor R1, and outputs the resultant as a high frequency voltage signal.
In the high frequency detection device 510 shown in FIG. 14, the current transformer portion 512, the capacitor portion 514 and wires are formed on a printed circuit board in the form of printed wiring, and it is therefore possible to suppress a situation in which variations occur in the shapes of the current transformer portion 512, the capacitor portion 514 and the wires. Accordingly, it is possible to suppress a situation in which variations occur in the values detected by the high frequency detection device 510.
There is, however, a problem in that the electrostatic capacity of the capacitor C1 formed by the ring-shaped wiring 514a of the voltage detection printed circuit board 517 is limited. Specifically, the electrostatic capacity of the capacitor C1 is proportional to the length of the through hole of the ring-shaped wiring 514a, or in other words, the thickness of the voltage detection printed circuit board 517, but because the thickness of the voltage detection printed circuit board 517 cannot be increased due to cost and technical reasons, it is difficult to increase the electrostatic capacity of the capacitor C1.
For example, the high frequency voltage generated in the power transmission body 511 can reach approximately several thousand volts because high voltage is used in plasma processing. The high frequency voltage signal output by the voltage conversion circuit 515 is input into the computing unit 520, and thus it is necessary to adjust the voltage level of the high frequency voltage signal output by the voltage conversion circuit 515 to approximately several volts. In other words, it is necessary to cause the voltage conversion circuit 515 to attenuate the high frequency voltage generated in the power transmission body 511 by a factor of approximately 1000 before outputting the signal. Accordingly, it is necessary to reduce an electrostatic capacity C1 of the capacitor C1 to approximately 1/1000 of an electrostatic capacity C2 of the capacitor C2. The voltage conversion circuit 515 can thereby attenuate the voltage level of the divided voltage generated at the connection point between the capacitor C1 and the capacitor C2 to approximately 1/1000 of the high frequency voltage generated in the power transmission body 511 and output the signal.
Too large a combined electrostatic capacity of the capacitor C1 and the capacitor C2 is not preferable because a current branching to the capacitor C1 of the high frequency current flowing through the power transmission body 511 will be large. Conversely, if the combined electrostatic capacity is too small, the current branching to the capacitor C1 will be too small, lowering the detection accuracy of the high frequency voltage signal. Accordingly, it is necessary to design the combined electrostatic capacity of the capacitor C1 and the capacitor C2 such that the current branching to the capacitor C1 falls within a predetermined range.
As described above, it is necessary to design the electrostatic capacities C1 and C2 of the capacitor C1 and the capacitor C2 taking the attenuation factor and the combined electrostatic capacity into consideration. For example, the electrostatic capacity C1 of the capacitor C1 is set to approximately 0.5 to 1 pF, and the electrostatic capacity C2 of the capacitor C2 is set to approximately 500 to 1000 pF. Also, the computing unit 520 usually includes a voltage follower circuit (whose input impedance is high) provided at an input terminal thereof, and thus hardly any current flows to the resistor R1 for adjustment. A current may, of course, flow depending on the design of the computing unit 520. For example, in the case where a resistor is provided at an input terminal of the computing unit 520 between the input terminal of the computing unit 520 and the ground potential, some current flows to the resistor R1 as well. Accordingly, adjustment by the resistor R1 becomes possible.
The electrostatic capacity C1 of the capacitor C1 can be calculated by the following equation:C1=2π∈·l/(ln(b/a))  (1),
where the outer diameter of the power transmission body 511 is indicated by a, the inner diameter of the ring-shaped wiring 514a is indicated by b (see FIG. 15(a), and the height of the ring-shaped wiring 514a (the length of the through hole; see FIG. 15(c)) is indicated by l.
Note that π indicates the circumference ratio, ∈ indicates the dielectric constant, and ln indicates the natural logarithm. The ring-shaped wiring 514a has a gap between through holes, and thus the actual electrostatic capacity will be smaller than the electrostatic capacity C1 calculated using the Equation (1) given above.
The high frequency voltage generated in the power transmission body 511 is high, and thus a dielectric breakdown will occur if the distance between the power transmission body 511 and the ring-shaped wiring 514a is small. In order to avoid the dielectric breakdown and to be safe, it is desirable to design the distance between the power transmission body 511 and the ring-shaped wiring 514a to have leeway. For example, it is desirable to set the distance to 20 mm or greater (the required distance varies depending on the use conditions). In the case where the outer diameter a of the power transmission body 511 is 20 mm, for example, in order to set the distance between the power transmission body 511 and the ring-shaped wiring 514a to 20 mm, the inner diameter b of the ring-shaped wiring 514a is required to be set to 60 mm. In this case, in order to obtain an electrostatic capacity C1 of the capacitor C1 of 0.5 pF, from the Equation (1) given above, the height l of the ring-shaped wiring 514a is required to set be to about 9.9 mm. Also, if the distance between the power transmission body 511 and the ring-shaped wiring 514a is set to a value greater than 20 mm, the height l of the ring-shaped wiring 514a is required to be set to an even greater value.
If the distance between the power transmission body 511 and the ring-shaped wiring 514a is reduced, the height l of the ring-shaped wiring 514a can be reduced. However, for example, when the outer diameter a of the power transmission body 511 is 20 mm and the distance between the power transmission body 511 and the ring-shaped wiring 514a is 10 mm, in order to obtain an electrostatic capacity C1 of the capacitor C1 of 0.5 pF, from the Equation (1) given above, the height l of the ring-shaped wiring 514a is required to be set to about 6.2 mm. Also, in the case of increasing the electrostatic capacity C1 of the capacitor C1 to a value greater than 0.5 pF, the height l of the ring-shaped wiring 514a is required to be set to an even greater value.
When consideration is given to the reduction of the electrostatic capacity due to the gap between through holes, the height l of the ring-shaped wiring 514a is set to an even grater value. However, the voltage detection printed circuit board 517 can currently have a maximum thickness of approximately 5 mm. Accordingly, it is not possible to obtain a desired electrostatic capacity of the capacitor C1 depending on the use conditions.